In the case of a power plant with a steam generator, the hot gas generated during the combustion of a fossil fuel is used to evaporate a flow medium in the steam generator. To evaporate the flow medium, the steam generator has steam generator tubes, which are heated with hot gas to cause evaporation of the flow medium conducted therein, generally water. The steam supplied by the steam generator can for example be provided for a connected external process or to drive a steam turbine. If the steam drives a steam turbine, a generator or production machine is generally operated via the turbine shaft of the steam turbine.
A steam generator can be conceived according to a range of design principles. In a continuous steam generator the heating of a number of steam generator tubes, which together form the gas-tight enclosing wall of the combustion chamber, results in total evaporation of a flow medium in the steam generator tubes in one pass. After evaporation the flow medium is generally fed to the superheater tubes arranged after the steam generator tubes, where it is superheated.
In contrast to a natural circulation steam generator, a continuous steam generator is not subject to pressure limitation, so that it can be designed for live steam pressures far above the critical pressure of water (pcritical=221 bar). A high live steam pressure and a high live steam temperature favor a high level of thermal efficiency and therefore lower CO2 emissions from a fossil-fuel heated continuous steam generator.
Generally in a continuous steam generator the side walls of the combustion chamber, viewed in the direction of flow of the hot gas, are divided into a number of throughflow segments formed by evaporator heating surfaces. The steam generator tubes, each welded together in a gas-tight manner and able to be flowed through from bottom to top, are assembled in each of the throughflow segments such that they can each be subjected to the action of a flow medium in a parallel manner. To this end an intake collector acting as a distributor is connected before each throughflow segment and an outlet collector is connected afterward. Such a configuration allows reliable pressure compensation between the steam generator tubes of a throughflow segment that are connected in a parallel manner and thus particularly favorable distribution of the flow medium as it flows through the steam generator tubes.
In the case of the continuous steam generator known for example from WO 01/01040 A1, the throughflow segments arranged in the side walls of the combustion chamber are connected in series on the flow medium side such that the flow medium flows through them in the sequence of their arrangement along the flow path provided for the hot gas inside the combustion chamber. In other words, the flow medium provided for the operation of the continuous steam generator, having as yet no steam element and being comparatively cold, is first fed to the first throughflow segment of the side wall, viewed in the direction of flow of the hot gas. The first intake collector assigned to this segment distributes the flow medium to the steam generator tubes that can be subjected to its action in a parallel manner, in which a first evaporation of the flow medium takes place. The water-steam mixture thus generated is collected in an outlet collector arranged after the first throughflow segment and fed via a line or a line system to the intake collector of the second throughflow segment, viewed in the direction of flow of the hot gas, where further heat is supplied to the flow medium and it is evaporated further. Reference is therefore made to a first and second evaporator stage, which can optionally be followed by still further evaporator stages. The outlet collector of the first evaporator stage can alternatively also be configured such that it acts as the intake collector to the second evaporator stage at the same time.
A preheater (economizer) is generally connected before the first evaporator stage on the flow medium side, utilizing the residual heat of the hot gas leaving the combustion chamber via a gas train connected afterward on the hot gas side to preheat the flow medium to be evaporated. This increases the overall efficiency of the continuous steam generator. The preheater does not however represent an evaporator stage, as the flow medium leaving it does not as yet have a steam element.
In steam states that place high demands on design, in particular at live steam temperatures of around 600° C., which are sought and also achieved for a high level of thermal efficiency, the problem of material fatigue arises. The high level of thermal loading means that comparatively large regions of the side walls enclosing the combustion chamber have to be cooled particularly efficiently. To this end, in addition to smooth tubes arranged in a spiral manner, vertically oriented steam generator tubes, also provided with internal ribs for example, can be provided, with which a particularly efficient and regular heat transfer to the flow medium conducted in them can take place due to the wetting of the internal wall of the tube with a deposited fluid film. Comparatively low wall temperatures are achieved as a result.
If still higher live steam temperatures of up to around 700° C. are sought, such tube cooling designs alone do not suffice for reliable long-term operation with the known steam generators. Instead particularly high quality and expensive materials are required in this instance during the manufacture of the steam generator tubes, said materials having to undergo a subsequent heat treatment after welding at the assembly site of the steam generator. The associated assembly cost is so high that continuous steam generators designed for such demanding steam states have not yet been produced.